Well. I say overnight. I changed my work schedule and I was not able to do a water change for a week or so. I came home the other day to find one of my guppies bloated and pine-coning. I put him in quarantine but he did not make it. I did a 25% water change but my nitrate levels are still at 20ppm. This confuses me because I have gone longer without water changes in the past.